1. Field of the Invention
The invention relates to semiconductor devices, and more specifically to semiconductor-on-insulator (SOI) structures and methods for fabrication thereof to improve metal oxide semiconductor field effect transistor (MOSFET) performance in complementary metal oxide semiconductor (CMOS) devices.
2. Description of the Related Art
The migration and segregation of mobile dopants, such as boron, to oxide materials is a well known phenomenon in semiconductor structures. For example, the migration and segregation of boron to oxide isolation regions may cause narrow width effects in semiconductor channels which in turn may impact a semiconductor device threshold voltage. Within the context of devices fabricated within semiconductor-on-insulator (SOI) structures, the impact of boron migration and segregation may be even more severe since there is typically also a buried oxide interface located below a semiconductor device. In ultra-thin SOI (UTSOI) devices, the boron migration and segregation effect may become even more extreme since an oxide interface typically abuts a boron doped extension region.
A massive loss of boron into an underlying buried oxide (BOX) layer may lead to a high parasitic resistance and a compromised semiconductor device performance. Loss of boron from a substrate into a buried oxide (BOX) layer can also affect operation of devices in which substrate doping is used as a back gate, or where substrate doping is used to set a threshold voltage as in the case of fully-depleted devices with a thin buried oxide (BOX) layer.
A potential solution to boron migration might be the use of a laser or flash anneal process for boron activation, but this solution severely restricts the process flow, and requires that a boron dopant in a semiconductor-on-insulator (SOI) structure never be exposed to a conventional spike anneal once the boron dopant is in place. This solution may lead to increased complexity in device integration, and to added constrains in device design.
Novel buried oxide (BOX) layer structures that may be used within semiconductor-on-insulator (SOI) structures are disclosed in the semiconductor fabrication art.
For example, Hsu, in U.S. Pat. No. 5,468,657, teaches a method for nitridation of a buried oxide (BOX) layer within a semiconductor-on-insulator (SOI) structure formed using a separation by implantation of oxygen (SIMOX) method. The nitridation method uses nitrogen implantation to the same depth to which oxygen is implanted during the separation by implantation of oxygen (SIMOX) method.
In general, one of the drawbacks of the SIMOX process is a high back-interface charge at the oxide/Si interface. The presence of back interface charge can lead to mobility degradation (and hence reduced drive current) in the channel of CMOS transistors fabricated on SIMOX substrates. The degradation is especially severe in CMOS devices fabricated on SIMOX substrates where the active Si layer is thin (<20 nm) as is the case for fully-depleted CMOS fabricated on ultra-thin SOI.
Semiconductor structures are certain to continue to increase in density and decrease in dimension. As a result of these density and dimensional trends, desirable are additional semiconductor structures and methods for fabrication thereof that impede boron or other mobile dopant migration and segregation to an oxide material, such as a buried oxide (BOX) material.